Telecommunication system for transmitting full motion video

ABSTRACT

An improved telecommunication system is provided which allows the transmission of full motion color video, voice and data over a communications link formed of a twisted pair of wires of the type currently utilized only for voice and data communications. The signal is transmitted over the twisted pair link as a broadband signal up to a distance of six thousand feet. The twisted pair link exhibits a known impedance to the signal, depending upon the lengths of the twisted pairs of wires in the link. As the signal is received from the twisted pair link it is amplified and an impedance proportional to the impedance of the twisted pair link and vectorially opposite thereto is impressed thereon. A unity gain is achieved whereby the signal degrading effects of the impedance of the twisted pair link are reversed. This results in the accurate reproduction of a video signal over a link including significant lengths of conventional twisted pairs of telephone wires. Moreover, because the compensation for telephone line twisted pair impedance allows the accurate reproduction of a broadband signal, a plurality of different video, telephone and data signals can be multiplexed together and transmitted simultaneously over the same twisted pair link.

BACKGROUND OF THE INVENTION

1. Description of the Prior Art

The present invention is a system for transmitting multiplecommunication signals, including at least one video signal,simultaneously over a link formed of a twisted pair of telephone wires.

2. Field of the Invention

In conventional telecommunication systems the transmission of fullmotion video images has heretofore been possible only over communicationlinks especially adapted for broadband transmission, as contrasted withthe bandwidth of signals employed in the transmission of telephonecommunications. The transmission of voice signals in telephonecommunications requires a bandwidth of about 4 kilohertz. This bandwidthwill accommodate frequencies within the range of the human voice and thefrequencies of dial and pulse code tones used for establishingconnections.

To meet the requirement for providing telephone service to communitiesthroughout this country and throughout the world an infrastructure oftelephone lines has been established from telephone central offices tosubscriber locations. In many countries including this country suchtelephone lines are formed of lengths of twisted pairs of insulatedcopper wires of 16, 22, 24 or 26 gauge (AWG). Incalculable lengths ofsuch twisted pairs of telephone wires have been installed by suspensionfrom telephone poles and in buried conduits to link subscriber locationsto telephone central offices, which in turn are linked throughout thecountry and throughout the world by telephone communications networks.

Due to the impedance of a link formed of a twisted pair of telephonewires there is always an attenuation of the electrical communicationsignals transmitted over these wires. The extent of this attenuationdepends upon the length of the twisted pair link. That is, the longerthe length of the twisted pair link the greater will be the totalimpedance between the central office and the subscriber's handset.

It is desireable in conventional telephone systems to create auniformity in the attenuation of a signal from a subscriber location sothat all of the telephone signals reaching a central office can beuniformly amplified and processed through the same type of commoncarrier equipment. The attenuation in a conventional link formed of atwisted pair of telephone wires below a prescribed frequency can bereduced and the lost frequency characteristic made nearly flat, byinserting series inductance periodically. This practice of insertinginductance periodically is termed loading. It has been conventionalpractice to load twisted pairs of telephone wires at intervals of sixthousand feet where the wires are designed to conduct frequencies offrom 0 to 4 kilohertz. The loading coils are normally located inmanholes and in elevated equipment boxes on telephone poles. Theconventional loading practice provides a relatively uniform signal levelat telephone central offices from subscriber locations located atvarying distances from the central offices.

While conventional telephone lines formed of twisted pairs of wire aresuitable for the transmission of voice communications between telephonehandsets and also for data communications, such as between computers andfacsimile machines, the transmission of video signals over conventionaltelephone lines including twisted pair links has heretofore beenimpractical. Unlike voice and data communications which require abandwidth of only about 4 kilohertz, the transmission of full motionvideo signals, such as television signals, requires a bandwidth of aminimum of 4.5 megahertz. In telephone communications systems in thiscountry the Federal Communications Commission requires telephonecommunication carriers to allocate a bandwidth of 6 megahertz for thetransmission of television signals. In other countries, such as inEurope, a bandwidth of 7.5 megahertz is required.

The reason that a video signal requires a bandwidth far greater than avoice signal is because of the multitude of points which must be scannedin a horizontal scanning raster in order to convey a video picture. Atthe end of each scan a blanking pulse is generated with respect to thevideo information to be reproduced, and a line synchronizing pulse of apolarity opposite to the video information is generated during thisblanking pulse. The line pulses synchronize the individual horizontalscanning lines. Similarly, it is necessary to synchronize the field orvertical scans. This is done by another pulse train which occurs duringthe field retrace time.

In conventional full motion or "fast scan" television transmission, thetime interval from the start of one horizontal line to the next is 63.5microseconds. Each complete frame consists of two interlaced fields. Theframe rate is 30 per second and there are 525 scanning lines per frame,of which about 93% are visible because of the loss of time during thefield blanking pulse. To make a horizontal resolution approximatelyequal to a vertical resolution, a minimum bandwidth of about 4.5megahertz is required. This frequency range and above will hereinafterbe referred to as a broadband frequency spectrum, as contrasted with thenarrow band frequency spectrum of 4 kilohertz which is employed in voiceand data transmission.

Conventional lines currently employed in the infrastructure of telephonecommunications have heretofore been considered unsatisfactory for thetransmission of video signals due to the degradation which video signalsexperience in passing over a conventional twisted pair of telephonewires. For a length of more than about one thousand feet, the horizontalsync pulses of a video signal lose their definition and becomeintermixed with the video data. As a consequence, the receiver cannotdistinguish between horizontal scan lines. Also, the portions of thesignals near the upper and lower ends of the frequency band experiencefar greater degradation than the portions of the signals near the middleof the band. Since color information is transmitted at the upper end ofthe frequency band of a video signal, color quickly becomes lost intransmission of a video signal over links including conventional twistedpairs of telephone wires. Thus, the transmission of full motiontelevised video signals, especially color video, has heretofore requiredbroadband carrier facilities. The existing infrastructure of telephonecommunications lines has heretofore been considered unacceptable for thetransmission of full motion video signals.

SUMMARY OF THE INVENTION

A primary object of the present invention is to provide atelecommunication system employing the medium of a link of one or morelengths of twisted pairs of telephone wires for use in transmitting fullmotion video signals. The telecommunication system of the inventionthereby allows a television camera to transmit a video signal over aconventional twisted pair of telephone wires.

By providing a system for transmitting video signals over communicationlinks employing conventional twisted pairs of telephone wires, theinvention allows video signals to be transmitted over the existinginfrastructure of telephone communication lines that are in place andoperating throughout virtually all population centers. The inventiondispenses with the need for special purpose broadband carrier facilitiesto be routed into a building structure in order to transmit a videosignal from that location.

In one broad aspect the present invention may be considered to be atelephone communication circuit for transmitting video signalscomprising: video signal initiation means for providing a video signalof at least 4.5 megahertz bandwidth, line driver means coupled to thevideo signal initiation means to amplify the video signal, a link formedof a twisted pair of unloaded telephone wires having opposite ends oneof which is coupled to receive the amplified video signal, symmetricalvideo transformer means coupled to the other of the ends of the twistedpair link to receive and amplify the video signal, correcting amplifiermeans coupled to receive the video signal from the symmetrical videotransformer and to impress thereon an impedance proportional to theimpedance in the twisted pair link and in vector opposition thereto tothereby correct the video signal for impedance degradation in saidtwisted pair link, and a video receiver coupled to the correctingamplifier means for receiving the corrected video signal.

In another broad aspect the invention may be considered to be animprovement in a telephone communication circuit including a twistedpair link comprised of an unloaded twisted pair of wires of a knownlength and having first and second ends. The improvement of theinvention is comprised of a video signal provision means for providing avideo signal having a bandwidth of at least 4.5 megahertz, line drivermeans interposed between the video signal provision means and the firstend of the twisted pair link to amplify the video signal, whereby thetwisted pair link exhibits a known impedance thereto, unity gainamplification means coupled to the second end of said twisted pair linkto reproduce the video signal with a compensating impedance impressedthereon that is proportional and opposite to the known impedance, and avideo receiver coupled to receive the video signal with the compensatingimpedance impressed thereon.

The principle of operation of the invention requires the pairs oftelephone wires in the twisted pair link to be unloaded. This is easilydone where the pair of wires is no greater than six thousand feet inlength, since loads on such wires are normally imposed only at thesubscriber location and at the telephone central office. Any loading onthe twisted pair of telephone wires must be removed in order for theinvention to function properly. If the twisted pair link is greater thansix thousand feet, line amplifiers have to be employed at intervals ofno greater than six thousand feet. The twisted pair link may either beformed of a single twisted pair of wires extending between two stations,the aggregate length of subscriber lines connected between subscriberlocations and a common central office, or the aggregate length ofsubscriber lines connected between subscriber locations and differentcentral offices.

In the absence of loading, a conventional twisted pair of telephonewires will exert a known impedance on a video signal transmitted overthose wires. Since impedance is a transmission factor with a vector, itis possible to impose a corresponding, opposite impedance on thereceived signal before the signal is passed to the video receiver fordisplay. The effects of impedance on the video signal are therebyreversed. Therefore, although the horizontal sync pulses of a videosignal are degraded, as are the information be ring portions of thevideo signal at the upper and lower ends of the video signal band, thecorrective impedance impressed on the received video signal has thegreatest corrective effect on these portions of the video signal. As aconsequence, once the opposing impedance has been impressed upon thereceived video signal, the signal is quite accurately reconstructed toits original form that it had upon its arrival at the transmitting endof the twisted pair of telephone wires.

Because it is possible to fully reconstruct a video signal, aconventional telephone twisted pair link is no longer limited to thenarrow 4 kilohertz frequency band of voice and data transmission. To thecontrary, broadband or wide band signals may be transmitted overconventional twisted pairs of telephone wires. The video signaltransmitted over the telecommunication circuit of the inventionpreferably is provided by a video signal initiation means that has abandwidth of at least about 6 megahertz. Thus, a video signal conformingto the Federal Communication Commission television channel requirementsfor monochrome and color television transmission may be transmittedaccording to the invention.

The correcting amplifier means which impresses the corrective impedanceupon the received video signal is preferably comprised of a receivesignal emitter follower amplification circuit and resistor-capacitorcombinations which are selectively and alternatively strapped into thereceive signal emitter follower amplification circuit as determined bythe length of the twisted pair link. The correcting amplifier means isalso comprised of phase adjustment means for applying a phase adjustmentto the video signal. This phase adjustment means is formed of a phaseadjustment amplifier circuit and resistor-capacitor-inductorcombinations which are selectively and alternatively strapped into thephase adjustment amplifier circuit as determined by the length of thetwisted pair link.

The line driver means is preferably comprised of a gain selection meansincluding a transmit signal emitter follower amplification circuit andresistor-capacitor combinations which are selectively and alternativelystrapped into the transmit signal emitter follower amplification circuitas determined by the length of the twisted pair link. By providing theline driver means with selected resistor-capacitor combinations, thesystem can be adjusted so that the video signal is degraded moreuniformly across its bandwidth as it is transmitted through the twistedpair of telephone wires, and is not excessively degraded at the upperand lower ends of the video signal frequency band. It thus is moreeasily corrected and reconstructed by the symmetrical video transformermeans and the correcting amplifier means.

To take further advantage of the capability of broadband transmissionnow possible utilizing conventional twisted pair links according to theinvention, preferred embodiments of the invention involve not only thetransmission of a single video signal over a conventional twisted pairlink, but also the multiplexing of a plurality of signals over aconventional twisted pair link. The plurality of signals are frequencymodulated together at the transmitting end and demodulated at thereceiving end of the telecommunication circuit. Indeed, a plurality ofvideo signals, a plurality of voice signals, a plurality of datasignals, and various combinations of video, voice and data signals canbe transmitted over a single conventional twisted pair link comprised oftelephone wires designed for use on an ordinary telephone handset usingmultiplexing techniques.

In such a multichannel system according to the invention thetelecommunication circuit is comprised of not only a video signalinitiation means but also at least one voice band signal initiationmeans for providing a voice band signal. Such a signal has a bandwidthof about 4 kilohertz. The system is further provided with multiplexingmeans coupled to receive the video signal from the video signalinitiation means and the voice band signal from the voice band signalinitiation means and to transmit the video signal and the voice bandsignal simultaneously through the line driver means and over the twistedpair link. In addition to the video receiver a voice band receiver iscoupled to the correcting amplifier means for receiving the voice bandsignal.

The system of the invention may be utilized to transmit signalsmultiplexed together over a 10 megahertz bandwidth. Thus, for example, avideo signal may be multiplexed together with a full duplex telephonevoice signal of 4 kilohertz and further additional full duplex datasignals, voice signals, or a combination of data and voice signals.Moreover, a plurality of video signals can be multiplexed together andtransmitted utilizing the system of the invention. Indeed, by utilizingquadrature amplitude modulation techniques a total of four differentvideo signals can be multiplexed together and transmitted over a singletwisted pair link simultaneously.

The invention may be described with greater clarity and particularitywith reference to the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of one embodiment of a full duplex,multiplexed telecommunication system constructed according to theinvention.

FIG. 2 is a block diagram of one of the transmitting terminals at one ofthe stations in the telecommunication system of FIG. 1.

FIG. 3 is a frequency diagram showing the different signals as modulatedby the transmitting terminal of FIG. 2.

FIG. 4 is a block diagram of one of the receiving terminals at one ofthe stations in the telecommunication system of FIG. 1.

FIG. 5 is a schematic diagram of the direct current voltage supply forboth the transmitting terminal of FIG. 2 and the receiving terminal ofFIG. 4.

FIG. 6 is a schematic diagram of a double balanced mixing circuitemployed in both the transmitting terminal of FIG. 2 and the receivingterminal of FIG. 4.

FIG. 7 is a schematic diagram of an amplifier oscillator circuitemployed in both the transmitting terminal of FIG. 2 and the receivingterminal of FIG. 4.

FIG. 8 is a schematic diagram of a bandpass filter circuit employed inboth the transmitting terminal of FIG. 2 and the receiving terminal ofFIG. 4.

FIG. 9 is a schematic diagram of a notch filter circuit employed in boththe transmitting terminal of FIG. 2 and the receiving terminal of FIG.4.

FIG. 10 is a schematic diagram of an oscillator circuit employed in boththe transmitting circuit of FIG. 2 and the receiving circuit of FIG. 4.

FIG. 11 is a schematic diagram of a broadband amplifier circuit employedin the transmitting terminals of the telecommunication system of FIG. 1.

FIGS. 12a and 12b are schematic diagrams of the line driver circuitemployed in the transmitting terminals of the telecommunication systemof FIG. 1.

FIGS. 13a and 13b are schematic diagrams of the symmetrical videotransformer circuit employed in the receiving terminals of thetelecommunication system of FIG. 1.

FIGS. 14a and 14b are schematic diagrams of the correcting amplifiercircuit employed in the receiving terminals of the telecommunicationsystem of FIG. 1.

FIG. 15 is a block diagram of a variation of the telecommunicationsystem of FIG. 1.

DESCRIPTION OF THE EMBODIMENT

FIG. 1 illustrates a telecommunication system indicated generally at 10.The telecommunication system 10 extends between a pair of stations 12and 14, each having a transceiver, which are coupled together by twistedpair links 16 each comprised of an unloaded twisted pair of telephonewires.

STATIONS

At each of the stations 12 and 14 of the telecommunication system 10there is a video signal source, indicated diagrammatically as a videocamera 18 and also a video receiver indicated diagrammatically as atelevision receiver or monitor 20. The video signal source 18 has asignal frequency bandwidth of at least 4.5 megahertz and preferably atleast 6.0 megahertz.

Each of the stations 12 and 14 is further comprised of a multiplexingunit 22 having a frequency modulating circuit 24 and a frequencydemodulating circuit 26. The video signal source 18 is connected throughthe frequency modulating circuit 24 of the multiplexing unit 22 to abroadband amplification circuit 28. The broadband amplification circuit28 is coupled to the video signal source 18 to receive the signaltherefrom to produce an amplified signal. The broadband amplificationcircuit 28 is illustrated schematically in FIG. 11.

The unloaded twisted pair links 16 each have a length no greater thansix thousand feet and a known impedance. The twisted pair links 16 arcoupled to receive the amplified broadband signal from the broadbandamplification circuit 28. The transmitters of each of the stations 12and 14 are also each equipped with a line driver circuit 19 havingcircuit portions 218 and 220 which are illustrated respectively inschematic form in FIGS. 12a and 12b.

The receivers in each of the stations 12 and 14 are each provided with asymmetrical video transformer circuit 30 that is coupled to the twistedpair of telephone wires 16 to receive and amplify the broadband signaltherefrom. Each symmetrical video circuit 30 includes circuit portions250 and 252 which are illustrated in FIGS. 13a and 13b respectively.Each of the stations 12 and 14 also includes a correcting amplifiermeans in the form of a correcting amplifier circuit 32. The correctingamplifier circuit 32 has circuit portions 258 and 260 which arerespectively illustrated in FIGS. 14a and 14b.

The correcting amplifier circuit 32 is coupled to the symmetrical videotransformer circuit 30 to provide an offsetting impedance to thebroadband signal. This offsetting impedance is proportional and inopposition to the known impedance of the twisted pair link 16. Theoffsetting impedance thereby negates the effect of the known impedanceof the twisted pair 16 on the broadband signal that passes over thetwisted pair 16. The video receiver 20 is coupled to receive thebroadband signal from the correcting amplifier circuit 32 through thedemodulating circuit 26 to extract a video signal therefrom.

As illustrated in FIGS. 1-4, the stations 12 and 14 of thetelecommunication system 10 are comprised of not only circuitry fortransmitting and receiving a video signal over a twisted pair link 16,but are each further comprised of additional signal sources forproviding additional telecommunication signals. These signal sources areindicated diagrammatically as a telephone handset 34 and four datatransmission sources, indicated diagrammatically as the datatransmitting modules 36, 38, 40 and 42 of computer modems. Each of thestations 12 and 14 also is provided with a telephone handset 34' anddata receiving modules 36', 38', 40' and 42' which are respectivelycoupled to the telephone 34 and the data transmitting modules 36, 38, 40and 42 of the other station through the multiplexing units 22.

The frequency modulating circuit 24 is coupled to receive the signalsfrom the signal sources 18 and 34-42 and to modulate those signals. Thebroadband amplifier circuit 28 is coupled to the frequency modulatingcircuit 24 to receive the modulated signals therefrom and to pass theamplified signals to the line driver circuit 19. The demodulatingcircuit 26 is coupled to the symmetrical video transformer 30 and to thecorrecting amplifier 32 to receive the broadband signal of FIG. 3therefrom. The demodulating circuit 26 demodulates the broadband signalto separately reproduce the video signal and transmit it to a videoreceiver 20 and to separately reproduce each of the additional signalsand transmit them to corresponding communication receivers 34', 36',38', 40' and 42'. The communication receiver 34' is a conventionaltelephone handset, while the receivers 34', 36', 38', 40' and 42' areall data receiving modules of computer data modems.

DC POWER SUPPLY

The modulating circuit 24 of the stations 12 and 14 are identical toeach other and are of the type depicted in block diagram form in FIG. 2.Power is supplied to both the modulating circuit 2 and the demodulatingcircuit 26 at each station by a common power supply which is illustratedschematically in FIG. 5. The direct current power supply is derived fromeither a 110 or 220 volt alternating current power source. Thealternating current input is provided on either lines 162 and 164 orlines 162 and 166, depending upon whether the available alternatingcurrent power supply is 110 or 220 volts. The alternating current supplyis provided as an input to power transformer 168, the secondary of whichis connected to a rectifier circuit 170. The diodes D1-D4 of therectifier circuit 170 are model IN4003 diodes. The rectified poweroutput is provided to a model LP 2400, 24 volt direct current regulator.The power supply circuit also employs two 101 microfarad capacitors 174;two 1 microfarad capacitors 176; and two 2200 microfarad capacitors 178.A 2 microhenry inductor 180 provides the regulated direct current powersupply to the modulating circuit 24 and the demodulating circuit 26 asthe B+ voltage through a diode 182.

MULTIPLEX SYSTEM

The video signal generating source 18 of FIG. 1 produces a compositevideo signal 43 which has a video channel output 44 and an audio channeloutput 46 that appear on separate BNC connectors 48, as indicated inFIG. 2. The composite video signal 43 is from 0.7 to 1.77 volts, peak topeak with a 6 megahertz bandwidth and 75 ohms impedance. The videocomponent 44 and the audio component 46 of the video signal 43 are mixedtogether in a mixing circuit 50. The individual circuit modules of themodulating circuit 24 and the demodulating circuit 26 are illustrated inFIGS. 6-10. The mixing circuit 50 is depicted schematically in FIG. 6and is used to mix the audio and video components 44 and 46 of the videosignal into a composite signal of 6 megahertz bandwidth. The mixercircuit 50 transmits that signal to an amplifier circuit 52 which isillustrated schematically in FIG. 7. The amplified output from theamplifier 52 is directed to a bandpass filter 64 which is illustratedschematically in FIG. 8. The bandpass filter 64 passes an output of 0 to8 megahertz bandwidth. This signal appears at 55 in FIGS. 2 and 3 and iscoupled to line 56 which leads to the broadband amplifier 28 of FIG. 1,that is illustrated schematically in FIG. 11.

The telephone signal initiating device 34 is a conventional RJ11 fullduplex telephone handset. The telephone signal from the handset 34 ispassed as an additional signal input on line 58 to a 4 kilohertzbandpass filter 60 of the type illustrated schematically in FIG. 9. Fromthe bandpass filter 60 the telephone signal is directed to a mixercircuit 50 of the same type employed to mix the audio and videocomponents 44 and 46 of the video input signal 43. However, the mixercircuit 50 which receives the filtered telephone signal is used tomodulate the telephone signal with a subcarrier frequency generated byan oscillator circuit 62, depicted schematically in FIG. 10. Thesubcarrier frequency generated by the oscillator circuit 62 for thetelephone communication signal is a signal of 8.06 megahertz frequency.Both the mixer circuit 50 and the oscillator circuit 62 are powered bythe B+direct current voltage supply derived from the power supplycircuit depicted schematically in FIG. 5.

The output of the mixer 50 that amplifies the telephone input signalfrom the telephone handset 34 is directed to a bandpass filter circuit64 of the type depicted schematically in FIG. 8. The output of thebandpass filter 64 is directed to another amplifier circuit 52 of thetype depicted in FIG. 7. The amplifier circuit 52 which is used toamplify the telephone signal has the same circuit configuration as theamplifier circuit 52 employed to amplify the video signal, but withcomponents of different values so as to produce an amplified output in adifferent band range.

The output from the amplifier circuit 52 which amplifies the telephonesignal is directed to another bandpass filter 60 of the type depicted inFIG. 9. The filtered and amplified telephone signal that was firstoriginated at the telephone handset 34 is passed as a modulated signal66 lying in the frequency range between 8.0 and 8.10 megahertz. Themodulated telephone signal 66 is combined with the filtered andamplified video signal 55 and passes on line 56 to the broadbandamplifier circuit 28, depicted schematically in FIG. 11.

A further additional signal initiating means may be the transmittingportion 36 of an RS 232 full duplex serial input data modem. The inputportion 36 of the RS 232 modem produces a data signal 68 having a 4kilohertz bandwidth. The additional data signal 68 is transmitted to abandpass filter circuit 60 of the type depicted in FIG. 9 to eliminateambient 60 cycle harmonics and other noise. The filtered data signal isthen transmitted to a mixer circuit 50 of the type depicted in FIG. 6,where it is mixed with a subcarrier frequency of 8.26 megahertz by anoscillator circuit 62 of the type depicted in FIG. 10. The modulateddata signal is then passed to a broadband filter circuit 64 of the typedepicted in FIG. 8, and then to an amplifier circuit 52 of the typedepicted in FIG. 7. The amplified data signal is then passed to a notchfilter circuit 60 of the type depicted in FIG. 9. The filtered andamplified data signal 70 lies in a frequency band between 8.2 and 8.4megahertz.

The other additional data signals are likewise generated by thetransmitting portions of RS 232 full duplex data modems, indicated at38, 40 and 42. These transmitting modem portions generate additionaldata signals indicated respectively at 72, 74 and 76. These data signals72, 74 and 76 are processed precisely in the same manner as the datasignal 68, with the exception that the subcarrier frequency generated bythe oscillator circuit 62 for each data signal is different. Thefiltered and amplified data signal 78 lies in a frequency band between8.6 and 8.7 megahertz. The filtered and amplified data signal 80 lies ina frequency band between about 8.9 and 9 megahertz, while the modulateddata signal 82 lies in a frequency band of between about 9 and 10megahertz.

When the modulated signals 55, 66, 70, 78, 80 and 82 leave themodulating circuit 24, they lie within a broad 10 megahertz bandwidth,as indicated in FIG. 3. The composite signal on line 56 is then passedto the broadband amplification circuit 28 of FIG. 11.

MIXER CIRCUIT

FIG. 6 schematically illustrates the mixer circuit 50. The mixer circuit50 is employed in the processing of the video signal 43 to mix the audioand video signal components 44 and 46. In the processing of the videosignal 43 the audio component 44 is passed as a shielded input on line84 to the coil 86 of a transformer circuit. The video input 46 is passedas a shielded input on line 88 to the center of a transformer coil 90.The transformer coil 90 is coupled to a rectifier circuit 92, the outputof which is connected to a transformer coil 94, the center of which isgrounded. The composite mixed output of audio and video appears on thetransformer coil 96 and is passed as an output signal on line 98.

The same mixing circuit 50 is employed to mix the telephone and datasignals with the subcarrier frequency generated by the oscillatorcircuit 62 of FIG. 10. In the processing of the telephone and datasignals the voice or data signal is provided as a shielded input on line84 while the carrier frequency from the oscillator circuit 62 isprovided as a shielded input on line 88. The data signal, modulated bythe carrier frequency from the oscillator circuit 62 appears as anoutput on line 98.

AMPLIFIER

The modulating circuit 24 also employs a separate amplifier circuit 52for each of the video, telephone and data signals. The input to theamplifier circuit 52 appears on line 112 and is passed through acapacitor 114. In the circuitry for amplifying the video signal thecapacitor 114 is a 0.1 microfarad capacitor, while in the circuitry foramplifying the voice and data signals the value of the capacitor 114 is0.22 microfarads. B+ voltage is provided on line 116 from the powersupply circuit of FIG. 5. The amplifier circuit 52 also includes a 56Kohm resistor 118, a 15K ohm resistor 120 and a 68K ohm resistor 122. AnNPN transistor 124 provides an amplified common emitter output on line126.

OSCILLATOR

The oscillator circuit 62 is depicted in detail in FIG. 10. Theoscillator circuit 62 employs a model 2N489 field effect transistor 100,a 16 picofarad capacitor 102, and resistors 104, 106 and 108 havingvalues of 470 ohms, 27K ohms and 330 ohms, respectively. The oscillatoroutput appears on line 110, which is connected to the input line 88 ofthe double balanced mixer circuit 50 of FIG. 6 for each of the telephoneand voice signal modulating circuit paths.

BANDPASS FILTER

Each modulating circuit 24 also includes a passive bandpass filter 64for each of the video, telephone and date signals. The bandpass filtercircuit 64 is illustrated schematically in FIG. 8. The input appears online 128. The broadband filter circuit 64 includes a tank circuit havingan inductor 130 and a variable capacitor 132 connected in parallel fromground to the signal path. The inductor 130 has a value of 27microhenrys while the capacitor 132 varies between 0 and 80 picofarads.The tank circuit output is coupled between two capacitors 134 of 22picofarads each, which together are coupled in parallel with a 47picofarad capacitor 136 between the signal input on line 128 and thefiltered signal output on line 138.

The output line 138 from the bandpass filter circuit 64 in the videosignal modulating circuit carries the amplified and filtered videosignal 43 from the video camera 18 as the mixed and amplified videosignal 55. The bandpass filter circuit 64 may be tuned to provide afrequency output 6 megahertz in bandwidth for those video systems wherevideo channels are allocated a 6 megahertz bandwidth. Alternatively, thebandpass filter 64 may be tuned to provide an output of 7.5 megahertzbandwidth for those carrier systems in which video channels areallocated a 7.5 megahertz bandwidth output. The bandpass filters 64employed in filtering the telephone and data signals, on the other hand,are tuned to provide an output having a bandwidth of between 1 and 2megahertz for each telephone signal 58 and each data signal 68, 70, 72,74 and 76.

NOTCH FILTER

Each modulating circuit 24 also filters each of the narrow frequencyband telephone and data signals from the handset 34 and from the datamodem transmitter section 36-42 through notch filter circuits 60 of thetype depicted in detail in FIG. 9. The input signal to each notch filtercircuit 60 appears on line 140 as the negative input to an operationalamplifier 142. The notch filter circuit 60 also includes resistors 144,each having a value of 3.3K ohms, resistors 146, each having a value of10K ohms, a resistor 148 having a value of 68K ohms, and capacitors 150of 0.33 microfarads each. The output of operational amplifier 142 isdirected as the negative input to another operational amplifier 152,which is coupled in a loop to operational amplifier 154. The output ofoperational amplifier 152 is also connected to the negative input ofoperational amplifier 156. The output of operational amplifier 142 isconnected also to the negative input of an operational amplifier 158.The operational amplifiers 142, 144, 152, 156 and 158 are all ICcomponents, model MC4301. The filtered output of the notch filtercircuit 60 appears on line 160.

The wave form of the composite multiplexed broadband signal outputproduced by each modulating circuit 24 is depicted diagrammatically inFIG. 3. The video signal indicated at 55 may be either 6 megahertz or7.5 megahertz in bandwidth. The telephone signal 66 lies in a bandwidthbetween 8 and 8.10 megahertz. The data signal 70 lies in the bandwidthbetween 8.2 and 8.4 megahertz. Data signal 78 lies in the 8.6 to 8.7megahertz band, while data signal 80 has a bandwidth between 8.9 and 9megahertz. Data signal 82 lies in a bandwidth between 9 and 10megahertz.

BROADBAND AMPLIFIER

The broadband amplification circuit 28 depicted in FIG. 11 is interposedbetween the camera 18 that provides the video signal 43 and the firstend 15 of the twisted pair of telephone wires 16 to amplify themultiplexed video signal 43, as well as the multiplexed telephone anddata signals. The broadband amplification circuit 28 receives theamplified and modulated video, telephone and data signals from theirrespective sources and produces an amplified broadband signal output.

The broadband amplifier circuit 28 includes a tank circuit having a 245microhenry inductor 190 and a variable capacitor 192 that can be variedbetween 0 and 80 picofarads. The output of the tank circuit is connectedbetween a pair of capacitors 134 which are coupled in parallel withcapacitor 136. The tank circuit and the capacitors 134 and 136 filterthe input signal on line 56 to amplify only the desired 10 megahertzbandwidth and filter out ambient RF signals that are likely to bepresent in an environment of computers and video processing equipment.

The isolated 10 megahertz band output is passed to a capacitor 194 whichserves as the input to a two stage amplifier. A model BC548 NPNtransistor 196 provides the first stage of amplification. The emitter oftransistor 196 is connected to a 15K ohm resistor 198 and to a 150 ohmresistor 200 at the base of a model BC324 PNP transistor 202 whichserves to correct for impedances. The collector of transistor 202 isconnected to a 1.5K ohm resistor 204 and the emitter receives B+ voltagethrough a 1K ohm resistor 206.

The second stage of amplification is formed by transistors 208 and 210which are coupled together. Both of the transistors 208 and 210 aremodel BC548 NPN transistors. Two transistors are employed in parallel tohandle the amount of current that is developed. The emitters of thetransistors 208 and 210 are coupled to ground through resistors 212 and204, respectively. Resistor 212 has a value of 3.9K ohms. The output ofthe second amplification stage is directed through a 75 ohm resistor 214and appears as an output on line 216. The broadband amplifier circuit 28ensures that all portions of the spectrum of the input frequency on line56 are linearly amplified in the output signal 216. The signal amplitudeof output signal 21 is thereby uniform.

LINE DRIVER

The output 216 of broadband amplifier circuit 28 is provided as an inputto circuit portions 218 and 220 which together form the line drivercircuit 19 and are depicted schematically in FIGS. 12a and 12b,respectively. The signal input line 216 to the line driver circuit 19appears in FIG. 12a. The line driver circuitry of FIGS. 12a and 12b iscomprised of a gain selection means including a transmit signal emitterfollower amplification circuit and resistor-capacitor combinations whichare selectively and alternatively strapped into the transmit signalemitter follower amplification circuit as determined by the length ofthe twisted pair link 16. The values of the resistors and capacitors areset forth in Table 1. The amplifier, diode and transistor modeldesignations are also set forth in Table 1.

                  TABLE 1                                                         ______________________________________                                        Component         Value/Designation                                           ______________________________________                                        12IC1 =           78M15CFAI                                                   12D1=             IN 4003                                                     12D2, 12D3, 12D4 =                                                                              IN 4148                                                     12D5 =            BZX97A9V1                                                   12D6 =            BZX83C2V7                                                   12D7 =            BZX83C5V5                                                   12D8 =            BZX83C6V8                                                   12C1, 12C5 =      0.22 microfarad, 100 volts                                  12C2 =            10 microfarads, 35 volts                                    12C3 =            2.2 microfarads 16 volts                                    12C4 =            22 microfarads, 15 volts                                    12C6 =            0.1 microfarad, 63 volts                                    12C7 =            1000 picofarads, 63 volts                                   12C8 =            22 microfarads, 16 volts                                    12C9 =            100 picofarads                                              12C10 =           68 picofarads                                               12C11 =           220 picofarads                                              12C12 =           150 picofarads                                              12C13 =           56 picofarads                                               12C14 =           270 picofarads                                              12C15 =           300 picofarads                                              12C16 =           82 picofarads                                               12C17 =           330 picofarads                                              12C18 =           2.2 microfarads, 16 volts                                   12C19, 12C22 =    10 microfarads, 3 volts                                     12C20, 12C21 =    22 microfarads, 16 volts                                    12C23 =           47 microfarads, 63 volts                                    12C24, 12C25 =    10 microfarads, 35 volts                                    12T1, 12T2 =      BC558B                                                      12T3, 12T7 =      BF311                                                       12T4 =            BC558C                                                      12T5, 12T6, 12T10 =                                                                             BC548B                                                      12T8, 12T9, 12T11 =                                                                             BC338.40SIE                                                 12T12 =           BC328.40SIE                                                 12R1 =            15 ohms                                                     12R2, 12R5 =      82 ohms                                                     12R3 =            100± ohms                                                12R4 =            130 ohms                                                    12R6 =            22 ohms                                                     12R7 =            22K ohms                                                    12R8 =            100K ohms                                                   12R9, 12R13 =     10K ohms                                                    12R10 =           1.69K ± 1% ohms                                          12R11, 12R18, 12R19 =                                                                           1.5K ohms                                                   12R12 =           560 ohms                                                    12R14 =           56 ohms                                                     12R15 =           2K ohms                                                     12R16, 12R23 =    2.2K ohms                                                   12R17 =           100 ohms                                                    12R20 =           150 ohms                                                    12R21 =           5.6K ohms                                                   12R22 =           2.7K ohms                                                   12R24, 12R27 =    681 ± 1% ohms                                            12R25, 12R29 =    820 ohms                                                    12R26 =           180K ohms                                                   12R28, 12R36 =    680 ohms                                                    12R30, 12R31, 12R34 =                                                                           18 ± 2% ohms                                             12R32, 12R33 =    24 ± 2% ohms                                             12R35 =           18 ohms                                                     12R37 =           2.2K ohms                                                   12R38 =           1.5K ohms                                                   12R39 =           18 ohms                                                     ______________________________________                                    

As illustrated in FIGS. 12a and 12b there are a number of bridgesprovided initially in a normally open condition. These bridges areindicated at 12A, 12B, 12J, 12K, 12L, 12M and at 12X, 12V, 12II, 12III,12VI, 12Xl, 12Vl and 12IIl. Contacts 12A, 12B, 12J, 12K, 12L, and 12Mare selectively strapped by soldering, while contacts 12II, 12III, 12V,12X, 12XI, 12Vl, 12VI, 12IIl, 12E, 12F, 12G and 12H are plug-in bridges.

The selective strapping of the bridges determines whichresistor-capacitor combinations are brought into the line driver circuit19. To illustrate, the resistors 12R14 through 12R21 and 12R38 and thecapacitors 12C9 through 12C16 may be selected in alternativecombinations as determined by the plug-in bridge contacts 12X, 12V,12II, 12Xl, 12Vl and 12IIl to change the gain of the amplifier 12T3. Theappropriate gain of amplifier 12T3 is dependent upon the length andimpedance of the twisted pair link 16.

By selecting the appropriate resistor-capacitor combination, saturationof the transistor 12T3 is prevented, depending upon the frequency bandto be transmitted. That is, the circuit configuration depicted in FIGS.12a and 12b is suitable for transmitting a bandwidth of from 6 to 10megahertz as received from the output line 216 of the broadbandamplifier circuit 28. Appropriate selection of the resistor-capacitorcombinations is made depending upon the bandwidth, as well as upon theimpedance of the twisted pair link 16. The output of transistor 12T3 online 230 is uniformly amplified over the frequency band selected and isprovided to the amplification stages of transistors 12T5-12T12 in FIG.12b.

Appendix A hereto sets forth exemplary soldered and plug-in contactconnections which are strapped into the circuit as appropriate fordifferent lengths and different gauges of wire for the twisted pair link16, as indicated. The conditions of the contacts for other lengths andgauges of twisted pair links is ascertained empirically.

From the line driver circuitry of FIGS. 12a and 12b the amplifieroutputs are connected to the first end 15 of the twisted pair oftelephone wires 16. The telephone wires employed in the twisted pairlinks 16 are conventional insulated copper wires of 16, 22, 24 or 26gauge which are used throughout the national telephone network forsupplying telephone service to subscriber locations from telephonecompany central offices. The twisted pair links 16 must be unloaded andmay be of a length dependent upon the gauge of the wire employed. Forexample, a twisted pair link 16 with polyethylene insulation may be upto 11,055 feet in length if 19 gauge wire is employed. A twisted pairlink 16 with polyethylene insulation may be up to 9,570 feet in lengthif 20 gauge wire is employed; up to 7,755 feet in length if 22 gaugewire is employed; and up to 5,775 feet in length if 26 gauge wire isemployed. Twisted pair links of even greater lengths may be utilized bymaximizing the efficiency of the components employed in the system.

SYMMETRICAL VIDEO TRANSFORMER

At the second end 17 of each twisted pair link 16 the signal, includingthe video signal originally provided at 43, appears as an input to thesymmetrical video transformer circuit 30 having circuit portionsdepicted at 250 in FIG. 13a and at 252 in FIG. 13b. The input to thesymmetrical video transformer circuit portion 250 in FIG. 13a isprovided on lines 254 and 256 from the second end 17 of the twisted pairof telephone wires 16. The broadband signal on lines 254 and 256 issignificantly degraded, but is reconstructed beginning with thesymmetrical video transformer video portions 250 and 252 of FIGS. 13aand 13b.

The symmetrical video transformer circuit portions 250 and 252 includecircuit components as listed in Table 2. The values of the resistor andcapacitor components are listed in Table 2, as are the part or modeldesignations of the diodes, transistors and the operational amplifiersrequired.

                  TABLE 2                                                         ______________________________________                                        Component         Value/Designation                                           ______________________________________                                        13ICI =           78M15CFAI                                                   13D1 =            IN 4003                                                     13D2, 13D4 =      IN 4148                                                     13D =             BZX83C3V3                                                   13C1 =            0.22 microfarads, 100 volts                                 13C2, 13C3, 13C7 =                                                                              10 microfarads, 35 volts                                    13C4 =            220 microfarads, 3 volts                                    13C5 =            47 farads, 3 volts                                          13C6 =            22 farads, 3 volts                                          13C8, 13C9, 13C10 =                                                                             Variable 25 to 500                                                            picofarads                                                  13T1 =            BC33840SIE                                                  13T2, 13T5, 13T6 =                                                                              BC548B                                                      13T3 =            BF311                                                       13T =             BF324SIE                                                    13R1 =            22 ohms, 15 volts                                           13R2, 13R20 =     15K ohms                                                    13R3, 13R6 =      24 + 2% ohms                                                13R4, 13R7 =      18 + 2% ohms                                                13R5, 13R8 =      20 + 2% ohms                                                13R9, 13R23 =     22 ohms                                                     13R10 =           120K ohms                                                   13R11, 13R21, 13R29 =                                                                           1.5K ohms                                                   13R12 =           27K ohms                                                    13R13 =           68K ohms                                                    13R14, 13R15 =    33K ohms                                                    13R16 =           390 ohms                                                    13R17 =           15 NTC ohms                                                 13R18 =           10 NTC ohms                                                 13R19 =           1K ohms                                                     13R22 =           47 ohms                                                     13R24 =           1.3K ohms                                                   13R25 =           10K ohms                                                    13R26 =           3.3K ohms                                                   13R27 =           2.7K ohms                                                   13R28 =           3.9K ohms                                                   13R30 =           75 ohms                                                     13R31 =           150 ohms                                                    ______________________________________                                    

The symmetrical video transformer circuit portion 250 of FIG. 13aincludes resistor-capacitor combinations which are selected forconnection into the symmetrical video transformer circuit 30 accordingto the impedance of the twisted pair link 16. This impedance, in turn,is governed by the length of the twisted pair of telephone wires formingthe link 16, as well as by the gauge of the wires.

The symmetrical video transformer circuitry 30 includes contacts 13A,13B, 13C, 13D, 13E, and 13F which are normally open, but which can bebridged by soldered connections as required. Other resistor-capacitorcombinations may be connected in circuit by plug-in bridges and areindicated at 13R, 13S, 13T, 13U, 13V, 13W and 13X. Appendix A heretosets forth the different resistor-capacitor combinations which areselectively and alternatively strapped into the symmetrical videotransformer circuitry, as determined for specific lengths and gauges ofthe twisted pair link 16. The appropriate resistor-capacitorcombinations for other length and gauges of twisted pair links 16 can bedetermined empirically.

The signal leaves the second or receiving end 17 of the twisted pairlink 16 and appears on lines 254 and 256 in the symmetrical videotransformer circuit portion 250 of FIG. 13a. An impedance matchingtransformer 266, part number G5603 is matched to the impedance of thetwisted pair link 16 and amplifies all of the information received onlines 254 and 256, including the data signals, the telephone signals,the audio component of the video signal and the degraded video componentof the video signal. The impedance matching transformer 266 amplifiesall of this information with a uniform gain. The transistors 13T3, 13T4,13T5 and 13T6 of FIG. 13b further amplify the received signal to buildup the signal level for correction.

CORRECTING AMPLIFIER

The correcting amplifier circuit 32 includes amplifying portions 258 and260, which are respectively set forth schematically in FIGS. 14a and14b. The correcting amplifier circuit 32 compensates for the impedanceof the twisted pair link 16. The correcting amplifier circuit portion258 is coupled to receive the amplified output from the impedancematching transformer 266 of the symmetrical video transformer circuit30. The correcting amplifier circuitry portions 258 and 260 provide anoffsetting impedance to the broadband signal transmitted thereto fromthe symmetrical video transformer circuit portion 260. This offsettingor compensating impedance is proportional and in opposition to the knownimpedance of the twisted pair ink 16 to thereby negate the effects ofthe known impedance on the broadband signal. Table 3 lists thecorrecting amplifier components showing amplifiers and transistors bymodel number and the values of resistors and capacitors.

    ______________________________________                                        Component         Value/Designation                                           ______________________________________                                        14IC1 =           78M15CFAI                                                   14T1 =            BC32840SIE                                                  14T2 =            BF311                                                       14T3, 14T5, 14T6 --                                                                             BC548BVAL                                                   14T4 =            BF324SIE                                                    14T7 =            BC558C                                                      14T8 =            BC548C                                                      14T9 =            BC558BSIE                                                   14D1 =            IN4003                                                      14D2 =            IN4148                                                      14D3 =            BZX55A11                                                    14D =             BZX55A9VI                                                   14C1 =            .22 microfarads, 100 volts                                  14C2 =            10 microfarads, 35 volts                                    14C4 =            330 picofarads                                              14C5 =            180 picofarads                                              14C6 =            82 picofarads                                               14C7 =            120 picofarads                                              14C8, C21 =       270 picofarads                                              14C9 =            56 picofarads                                               14C10 =           220 picofarads                                              14C11 =           470 picofarads                                              14C12 =           560 picofarads                                              14C13 =           390 picofarads                                              14C14 =           47 picofarads                                               14C15 =           22 picofarads                                               14C16 =           5.6 picofarads                                              14C17, 14C22 =    100 picofarads                                              14C18, 14C19, 14C20 =                                                                           Variable 15 to 230                                                            picofarads                                                  14C23, 14C24, 14C25 =                                                                           18 picofarads                                               14C26 =           33 picofarads                                               14C27, C34 =      10 microfarads, 35 volts                                    14C28 =           .15 microfarads                                             14C29 =           .1 microfarads                                              14C30 =           2200 picofarads, 63 volts                                   14C31, 14C32 =    22 microfarads, 16 volts                                    14C33 =           120 picofarads, 63 volts                                    14C35, 14C36 =    47 microfarads, 6.3 volts                                   14R1 =            15 ohms                                                     14R2 =            56K ohms                                                    14R3 =            470 10% ± ohms                                           14R4, R29 =       180 ohms                                                    14R5 =            15K ohms                                                    14R6, 14R16, 14R39 =                                                                            1K ohms                                                     14R7 =            68K ohms                                                    14R8 =            1.6K ohms                                                   14R9 =            47K ohms                                                    14R10 =           3K ohms                                                     14R11, 14R20 =    1.8K ohms                                                   14R12 =           56 ohms                                                     14R13, 14R27 =    2.2K ohms                                                   14R14, 14R37 =    3.9K ohms                                                   14R15, 14R28 =    33 ohms                                                     14R17, 14R34 =    10K ohms                                                    14R18, 14R25 =    47 ohms                                                     14R19, 14R30, 14R32 =                                                                           22K ohms                                                    14R21 =           100 ohms                                                    14R22 =           2.7K ohms                                                   14R23 =           6.8K ohms                                                   14R24, 14R41 =    68 ohms                                                     14R26 =           1.2K ohms                                                   14R31 =           470 ohms                                                    14R33 =           220K ohms                                                   14R35 =           3.3K ohms                                                   14R36 =           4.7K ohms                                                   14R38 =           47 ohms                                                     14R40 =           1.3K ohms                                                   14R42 =           680 ohms                                                    14R43 =           330 ohms                                                    14DR1 =           15 microhenrys                                              14DR2 =           15 microhenrys                                              14DR3 =           10 microhenrys                                              14DR4 =           33 microhenrys                                              ______________________________________                                    

The correcting amplifier circuitry portions 258 and 260 are comprised ofan emitter-follower amplifier circuit and a plurality of combinations ofresistors, capacitors and inductors. At least one of the combinations ofresistors, capacitors and inductors is coupled in the emitter followercircuit, and other of the combinations are selectively isolatedtherefrom in accordance with the impedance of the twisted pair link 16.These resistor-capacitor-inductor combinations are selectively coupledin circuit and isolated by means of normally open bridges 14A, 14B, 14C,14D, 14E, 14F, 14G, 14H, 14I, 14J, 14K and 14L which can be selectivelyclosed by soldering, and by normally open plug-in bridges 14R+, 14S+,14T, 14U, 14V, 14W, 14X, 14X+, 14Y, 14Y+, 14Z and 14Z+. The closures ofthese bridges for several selected lengths and gauges of differenttwisted pair of links 16 are set forth in Appendix A.

The amplified broadband video signal leaves the symmetrical videotransformer circuit portion 252 on a signal output line 268. The signaloutput line 268 is connected as an input to the correcting amplifiercircuit portion 258 in FIG. 14a. The bridging connections of FIG. 14aselectively connect resistors 14R10 through 14R18 and capacitors 14C5through 14C15 into the amplifying circuitry of the correcting amplifiercircuit portion 258 to partially compensate for the impedance of thetwisted pair link 16. The correcting amplifier circuit portion 260performs the phase adjustment necessary to complete the correction forthe impedance of the twisted pair link 16.

The signal output lines 270, 272 and 274 from the correcting amplifiercircuit portion 258 of FIG. 14a are connected as inputs to thecorrecting amplifier circuit portion 260 of FIG. 14b. The bridgingcontacts 14E, 14F, 14G, 14H, 14I, 14J, 14K, 14L, 14T, 14U, 14W and 14Vare used to strap in the resistor-capacitor-inductor combinations thatadjust the phase of the received broadband signal in order toreconstruct that signal.

The circuit portion 258 of the correcting amplifier circuit 32 serves asa trimming circuit and simulates a few turns of wire, so as tocompensate for any minor mismatch between the impedance matchingtransformer 266 and the impedance of the twisted pair link 16. Thecorrecting amplifier circuit portion 258 of FIG. 14a also functions as areceived signal emitter follower amplification circuit. Theresistor-capacitor combinations in the correcting amplifier circuitportion 258 are selectively and alternatively strapped into the receivedsignal emitter follower amplification circuit as determined by thelength of the twisted pair link 16. The correcting amplifier circuitportion 260 of FIG. 14b forms a phase adjustment means for applying aphase adjustment to the video signal. The circuit portion 260 employsresistor-capacitor-inductor combinations which are selectively andalternatively strapped into the phase adjustment amplifier circuit asdetermined by the length of the twisted pair of telephone wires 16.

Together the symmetrical video transformer circuit portions 250 and 252and the correcting amplifier circuit portions 258 and 260 form a unitygain amplification means that is coupled to the second end 17 of thetwisted pair link 16 to reproduce the video signal 55 with acompensating impedance impressed thereon that is proportional andopposite to the known impedance of the twisted pair link 16. Theimpedance impressed upon the signal by the correcting amplifier circuitportions 258 and 260 is equal to and cancels out the effects of the lineimpedance in the twisted pair link 16.

The final signal output line 276 from the correcting amplifier circuitportion 260 carries a broadband signal which is restored to theconfiguration of FIG. 3. The output signal on line 276 in the correctingamplifier circuit portion 260 is a fully reconstructed broadband signal,including all of the signal components of FIG. 3. The correctingamplifier circuitry reconstructs all of the signals, including the videosignal 55. That signal is fully reconstructed and complete withhorizontal sync pulses and color information in the video signal 55.

DEMODULATION

As the fully reconstructed composite broadband signal of FIG. 3 leavesthe correcting amplifier circuitry on line 276 of FIG. 14b, it is passedto demodulating circuitry indicated generally at 26 in FIG. 1. Thedemodulating circuitry 26 forms a frequency division demodulating meansthat is coupled between the impedance compensating means of FIGS. 13a,13b, 14a and 14b and the receivers for each of the signal components,indicated at 20, 34', 36', 38', 40' and 42' in FIG. 1. The demodulatingcircuitry 26 employs many of the same subcircuits of the modulatingcircuit 24.

The demodulating circuit 26 is depicted in block diagram form in FIG. 4.The demodulating circuit 26 removes the carrier frequency from each ofthe telephone and data channels and directs the signal component to anappropriate receiver. That is, as illustrated in FIG. 1, thereconstructed video signal 43', having reconstructed audio and videocomponents 44', 46', is directed to a television receiver 20. Thereconstructed telephone signal is directed to a telephone handset 34',which is identical to the telephone handset 34. Similarly, the datasignal from the data modem transmitting module 36 is directed to a datamodem receiving module 36'. The data signal from data modem transmittingmodule 38 is directed to a data modem receiving module 38'. The datasignal from data modem transmitting module 40 is directed to a modemdata receiving module 40'. The data signal from the data modemtransmitting module 42 is directed to a data modem receiving module 42'.

The system of FIG. 1 illustrates two stations 12 and 14 which areconnected directly to each other by a twisted pair link comprised of asingle pair of telephone wires 16. It is to be understood, however, thatthe system is equally applicable to stations connected in a nationwideor worldwide telecommunication network.

FIG. 15 illustrates such a system in which the station 12 is connectedby an unloaded twisted pair of telephone wires 316 to a telephonecentral office 318. The central office 318 is connected into anationwide common carrier telecommunication network including buried andoverhead coaxial cable, fiberoptics, laser, microwave, satellite andinfra-red telecommunications links, indicated generally at 324. Thestation 14 is likewise connected to a central office 320 by an unloadedtwisted pair of telephone wires 322. The central office 320 is alsoconnected into the same common carrier telecommunications network 324.

Thus, the invention may be applied to stations which are connected toeach other by a mere six thousand feet of a twisted pair of telephonewires as illustrated in FIG. 1, or to stations located thousands ofmiles apart and connected to each other by a twisted pair link includingcomponent lengths of pairs of twisted telephone wire 316 and 322 throughcentral offices 318 and 320 and a common carrier 324.

It is to be understood that the telecommunication system of theinvention is not necessarily connected to either a local or nationwidetelephone network. To the contrary, the stations 12 and 14, if locatedin physical proximity to each other, could be connected togetherdirectly by a single length of an unloaded twisted pair of telephonewires. The operation of the invention is the same whether the stations12 and 14 are connected directly together by a single length of aunloaded twisted pair of telephone wires 16, or by sections of twistedpairs 16 that extend to local telephone central offices, which areconnected together through a nationwide telephone network. Regardless ofthe distance over which the signals are transmitted beyond the twistedpair link, full reconstruction of the video signal 55 is possible.

The preferred embodiments of the telecommunication system of theinvention provide simultaneous, switchable communication of full motioncolor video with interactive voice and integrated data capabilities. Thesystem employs a multiplexer switch enhancer which accommodates,redistributes and manages the bi-directional transmission of full motioncolor images, as well as audio and data channels as desired.

While the telecommunication system of the invention has been illustratedas transmitting and receiving a single video signal, as well asadditional telephone and data signals, it is to be understood that thesystem is readily adaptable for the transmission and receipt of aplurality of video signals. That is, using quadrature amplitudemodulation techniques the system can accommodate up to four channels offull motion, color, video instead of the single video channel with atelephone and four data channels as described in the embodiment of FIGS.1-14b.

The telecommunication system of the invention provides analogcommunication enhancement and capability transmitted over a twisted pairof telephone wires. The system can be used for the twin wiretransmission of bi-directional video, voice, data, facsimile or telexedinputs and outputs from office to office, floor to floor, within abuilding or complex of buildings, and between buildings, studios andstations of distance up to six thousand feet over unloaded pairs oftelephone wires.

As illustrated in FIG. 1, the system can be constructed for full duplexoperation. In such a system each of the stations 12 and 14 includes fulltransmit and receive capabilities. The telecommunication system of theinvention may be either simplex, half duplex, or full duplex, and may beemployed for special purposes, such as home video, paper viewing, videoconferencing, security surveillance, medical and health care delivery,the monitoring of nuclear power plants, and innumerable otherapplications.

Undoubtedly, numerous variations and modifications of the invention willbecome readily apparent to those familiar with telecommunicationssystems. For example, the selectable contacts of Appendix A need not besoldered or plug-in connections as depicted in the drawing Figures, butcan be established through an EPROM. That is, the length and gauge ofthe wires in the twisted pair link can be programmed into the EPROM sothat the appropriate contacts will be established automatically.Accordingly, the scope of the invention should not be construed aslimited to the specific embodiments of the telecommunication systemdepicted and described, but rather is defined in the claims appendedhereto.

    __________________________________________________________________________    APPENDIX A                                                                    PLUG IN STRAPS AND SOLDER BRIDGES WHICH MUST BE                               CLOSED FOR VARIOUS WIRE LENGTHS, GAUGES AND TYPES                             LENGTH OF                                                                              LINE    SYM VIDEO   CORRECT                                          LINE     DRIVER 19                                                                             TRANS 30    AMP 32                                           __________________________________________________________________________    20 AWG - Poly-                                                                vinylchloride                                                                 Insulation (PVC)                                                              1320-2475 ft     B, A, W     G                                                2970-4125 ft     B, A, W     G, D, E, A                                       4620-5775 ft                                                                           A, III, VI                                                                            T, R, B, L, V, X                                                                          G, R+, D, E, A                                   26 AWG - Poly-                                                                ethylene                                                                      Insulation (PE)                                                               1320-2475 ft     B, A, W     G                                                2970-4125 ft     T, R, B, A, W                                                                             G, R+, D, A                                      4620-5775 ft                                                                           A, III, E, G                                                                          U, S, B, L  G, R+, D, A                                      22 AWG - PE                                                                   Insulated                                                                     1815-3300 ft     B, A, W     G                                                3960-5610 ft     B, A, W     G, D, C+, A                                      6270-7755 ft                                                                           A, B, III                                                                             T, R, B, L, A, W                                                                          G, R+, D, C+, A                                  20 AWG - PE                                                                   Insulated                                                                     2145-4125 ft     B, A, X     G                                                4950-6930 ft     B, A, X     G, D, C+, E, B, A                                7755-9570 ft                                                                           A, B, III, VI                                                                         T, R, B, L, A, V, W, X                                                                    G, R+, D, C+, E, B, A                            19 AWG - PE                                                                   Insulated                                                                     2475-4785 ft     B, A, X     G                                                5600-7920 ft     B, A, X     G, D, C+, E, B, A                                8745-11055 ft                                                                          A, B, III, VI                                                                         B, L, A, V, W, X                                                                          G, R+, D, C+, E, B, A                            26 AWG - Paper                                                                Insulated                                                                      990-1980 ft     B, A        G                                                2310-3300 ft     B, A        G, D, B                                          3630-4455 ft                                                                           A, III  B, L, W     G, D, B                                          22 AWG - Paper                                                                Insulated                                                                     1485-2805 ft     B, W        G                                                3465-4785 ft     B, W        G, D, A                                          5280-6600 ft                                                                           A, B, III                                                                             B, L, X     G, D, A                                          20 AWG - Paper                                                                Insulated                                                                     1650-2970 ft     B, A, W     G                                                3630-4950 ft     B, A, W     G, D, C+, A                                      5610-6930 ft                                                                           A, III, VI                                                                            B, L, A, X  G, D, C+, A                                      22 AWG - Poly-                                                                vinylchloride                                                                  Insulation (PVC)                                                              825-1485 ft     B, A, W     G                                                1815-2475 ft     T, R, B, A, W                                                                             G, D, C+, A                                      2805-3465 ft                                                                           A, B, III, F, G                                                                       T, R, B, L, A, X                                                                          G, D, C+, A                                      __________________________________________________________________________     Notes: R+ stands alone and D stands alone  should not be read as (R+D).       R+ was used as not to confuse "R" for resistor.                               C+ was used as not to confuse "C" for capacitor.                              III = ROMANS 3                                                                VI = ROMANS 6                                                            

I claim:
 1. A telecommunication circuit for transmitting video signalscomprising:video signal initiation means for providing a broadband videosignal of at least 4.5 megahertz band width, line driver means coupledto said video signal initiation means to amplify said broadband videosignal and transmit it intact, a link formed of a twisted pair ofunloaded telephone wires having opposite ends one of which is coupled toreceive said amplified broadband video signal, symmetrical videotransformer means coupled to the other of said ends of said twisted pairlink to receive and amplify said broadband video signal, correctingamplifier means coupled to receive said broadband video signal from saidsymmetrical video transformer and to impress thereon an impedanceproportional to the impedance in said twisted pair link and in vectoropposition thereto to thereby correct said broadband video signal forimpedance degradation in said twisted pair link and a video receivercoupled to said correcting amplifier means for receiving said correctedbroadband video signal.
 2. A telecommunication circuit according toclaim 1 wherein said broadband video signal as provided by said videosignal initiation means has a band width of at least 6.0 megahertz.
 3. Atelecommunication circuit according to claim 1 wherein said twisted pairlink is no greater than six thousand feet in length.
 4. Atelecommunication circuit according to claim 1 wherein said correctingamplifier means is comprised of a receive signal emitter followeramplification circuit and resistor-capacitor combinations which areselectively and alternatively strapped into said receive signalemitter-follower amplification circuit as determined by the length ofsaid twisted pair link.
 5. A telecommunication system according to claim4 wherein said line driver means is comprised of a gain selection meansincluding a transmit signal emitter follower amplification circuit andresistor-capacitor combinations which are selectively and alternativelystrapped into said transmit signal emitter follower amplificationcircuit as determined by the length of said twisted pair link.
 6. Atelecommunication circuit according to claim 1 wherein said correctingamplifier means is comprised of phase adjustment means for applying aphase adjustment to said video signal.
 7. A telecommunication circuitaccording to claim 6 wherein said phase adjustment means is comprised ofa phase adjustment amplifier circuit and resistor-capacitor-inductorcombinations which are selectively and alternatively strapped into saidphase adjustment amplifier circuit as determined by the length of saidtwisted pair link.
 8. A telecommunication circuit according to claim 1further comprising at least one voice band signal initiation means forproviding a voice band signal of about 4 kilohertz band width andmultiplexing means coupled to receive said broadband video signal fromsaid video signal initiation means and said voice band signal from saidvoice band signal initiation means and to transmit said broadband videosignal and said voice band signal simultaneously to said line drivermeans over said twisted pair link, and a voice band receiver coupled tosaid correcting amplifier means for receiving said voice band signal. 9.A telecommunication circuit according to claim 8 comprising a pluralityof voice band signal initiation means as aforesaid including at leastone telephone signal initiation means and at least one data signalinitiation means and a plurality of voice band receivers as aforesaiddedicated to receive each of said voice band signals.
 10. Atelecommunication circuit according to claim 8 wherein said multiplexingmeans is comprised of frequency division modulating means coupledbetween said signal initiation means and said line driver means andfrequency division demodulating means coupled between said correctingamplifier means and said receivers.
 11. A telecommunication systemcomprising:(a) a video signal source providing a broadband video signalhaving a frequency band width of at least 4.5 megahertz, (b) line drivermeans coupled to said video signal source to produce an amplifiedbroadband video signal; (c) a link comprised of an unloaded twisted pairof telephone wires having a known impedance and coupled to receive saidamplified broadband video signal intact from said line driver means, (d)a symmetrical video transformer coupled to said twisted pair oftelephone wires to receive and amplify said broadband video signaltherefrom, (e) correcting amplifier means coupled to said symmetricalvideo transformer to impress thereon an offsetting impedance to saidbroadband video signal emanating from said twisted pair link which isproportional and in opposition to said known impedance of said twistedpair link to thereby negate the effects of said known impedance on saidbroadband video signal from said video source, and (f) a video receivercoupled to receive said broadband video signal from said correctingamplifier means to extract a video image therefrom.
 12. Atelecommunication circuit according to claim 11 wherein said correctingamplifier means is comprised of an emitter follower amplifier circuitand a plurality of combinations of resistors, capacitors and inductors,and at least one of said combinations is selectively coupled in saidemitter follower circuit and other of said combinations are selectivelyisolated therefrom in accordance with the impedance of said twisted pairlink.
 13. A telecommunication circuit according to claim 11 wherein saidsymmetrical video transformer and said correcting amplifier means serveas a unity gain amplifier for signals passing over said twisted pairlink.
 14. A telecommunication circuit according to claim 11 furthercomprising at least one additional signal source for providing at leastone additional communication signal, and frequency modulating meanscoupled to receive said signals from said signal sources and to modulatesaid signals, and said line driver means is coupled to said frequencymodulating means to receive said modulated frequencies therefrom andfurther comprising demodulating means coupled to said correctingamplifier means to receive modulated signals therefrom, and todemodulate said modulated signals to separately reproduce said broadbandvideo signal and each additional communication signal, and separatecommunication receivers for each additional communication signal.
 15. Atelecommunication circuit according to claim 14 further comprising aplurality of additional signal sources as aforesaid including at leastone additional telephone signal and at least one additional data signal.16. A telecommunication circuit according to claim 14 furthercharacterized in that said additional communication signal is about 4kilohertz in bandwidth.
 17. A telecommunication circuit according toclaim 11 further characterized in that said twisted pair link is nogreater than six thousand feet in length.
 18. In a telecommunicationcircuit including a twisted pair link comprised of an unloaded twistedpair of wires of a known length and having first and second ends, theimprovement comprising:video signal provision means for providing abroadband video signal having a band width of at least 4.5 megahertz,line driver means interposed between said video signal provision meansand said first end of said twisted pair link to amplify said broadbandvideo signal intact and to impress a known impedance thereon, wherebysaid twisted pair link exhibits said known impedance, unity gainamplification means coupled to said second end of said twisted pair linkto reproduce said broadband video signal intact with a compensatingimpedance proportional and opposite to said known impedance impressedthereon, and a video receiver coupled to receive said broadband videosignal with said compensating impedance impressed thereon.
 19. Animproved telecommunication circuit according to claim 18 wherein saidunity gain amplification means is comprised of a symmetrical videotransformer at said second end of said twisted pair link to amplify saidbroadband video signal, and correcting amplifier means for impressingsaid compensating impedance on said broadband video signal once itemanates from said symmetrical video transformer.